The performance of an engine substantially depends on the valve timing of its intake and exhaust valves. Typically, a continuously variable valve timing (CVVT) apparatus is capable of changing such valve timing in accordance with driving conditions of the engine. Oil pressure of an engine is usually used to control a CVVT apparatus. Therefore, the performance of the CVVT apparatus is greatly influenced by the temperature and/or viscosity of the oil. The temperature and viscosity of the oil have a close relationship with the temperature of the engine (i.e., coolant temperature). Therefore, it is preferable that the coolant temperature is considered in controlling the CVVT.
An example of considering the coolant temperature in controlling CVVT is found in the Laid-open Publication of Japanese patent application 1999-36905. According to the disclosure therein, the coolant temperature takes a role as a factor in determining whether CVVT is to be operated, and to enable adjustment of an oil pressure duty ratio applied to the CVVT apparatus. Accordingly, a drawback of this system is that the CVVT apparatus is not operated when the engine is cold-started, e.g., when started at a temperature within a range of 0˜10° C. Another drawback is that excessive noxious exhaust gas is expelled into the atmosphere because the engine is not properly controlled while in a starting and warm-up mode.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention, and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known to a person skilled in the art.